Democrats countered that public commentary is hampered by secrecy laws and promised that more details about the programs' successes would be declassified as early as next week.

The NSA's director, Army Gen. Keith B. Alexander, briefed the House and Senate intelligence committees about his agency's data-collection efforts that were exposed last week by a former computer systems administrator.

FBI Director Robert S. Mueller III was closely questioned about the programs by skeptical lawmakers from both sides of the aisle.

Rep. Mike Rogers, chairman of the House Permanent Select Committee on Intelligence, said that terrorists already are modifying their communication habits after leaks about NSA's top-secret surveillance programs, which collect millions of telephone records and track foreign Internet activity on U.S. networks.

There are "changes we can already see being made by the folks who wish to do us harm, and our allies harm," Mr. Rogers said without providing details.

Rep. C.A. Dutch Ruppersberger of Maryland, the committee's ranking Democrat, said he expressed concern about Edward Snowden, the former NSA contractor who leaked information about the program and fled to Hong Kong.

"It seems unusual that he would be in China and asking for the protection of the Chinese government but we're going to investigate," Mr. Ruppersberger said.

In an interview published Thursday in Hong Kong's South China Morning Post newspaper, Mr. Snowden claimed that the U.S. has long been attacking a Hong Kong university that routes all Internet traffic in and out of the semiautonomous Chinese region.

Gen. Alexander gave the committee a closed-door briefing about NSA's surveillance programs, and lawmakers urged him to speedily make public whatever he can.

"The more we can get unclassified the more the public will understand," Mr. Ruppersberger said after the briefing.

He said he hopes that details about "at least 10" terrorist plots that had been uncovered with the programs' help could be revealed as soon as next week.

"It's in the advantage of people who really want this program understood to get those examples out," Mr. Rogers said.

But there are critics as well as defenders of NSA snooping on both sides of the aisle.

"We are on the verge of becoming a surveillance state," he said Thursday during a hearing.

"It seems clear that the government's activity exceeds the authority this Congress has provided, both in letter and in spirit," said Mr. Conyers, adding that he would introduce legislation Friday to impose restrictions and new oversight for surveillance programs.

Mr. Mueller attempted to assure lawmakers that the programs are limited, court-approved and overseen by Congress.

"The legality has been assured by the Department of Justice," Mr. Mueller said in his final scheduled appearance before Congress before he leaves office this year. He added that the Foreign Intelligence Surveillance Court "has ruled on these two programs, monitors these two programs and has assured the legality of the efforts undertaken in these two programs."

Top-secret documents about the two programs were posted online last week after being leaked to the news media by Mr. Snowden, a self-proclaimed whistleblower. One program involves the collection of telephone "metadata" about every U.S. phone call — information about who was calling whom, when and for how long.

Rep. Jason Chaffetz, Utah Republican, asked Mr. Mueller whether metadata included location information — records of where the telephone was when a call is made.

"That's a question I'd have to get back to you on. I had not thought about that," Mr. Mueller said.

Mr. Chaffetz noted that he had submitted his questions in advance, "so I could have a candid dialogue with you."

"It's terribly frustrating, sir," he said after Mr. Mueller promised to get back to him with the information. "This is an important discussion and dialogue, and I know I won't get an answer."

Among congressional members and staffers, the FBI has long been notorious for taking an inordinate length of time to respond to questions for the record arising out of the director's annual testimony.

Mr. Mueller promised Mr. Chaffetz that he would meet with him personally to answer what he said are "complex" questions about location data.

The Supreme Court recently held that a warrant should be required for police to attach a tracking device to a vehicle, but as Mr. Chaffetz pointed out, that would not be necessary if suspects could be tracked using their phone records.

• Jerry Seper contributed to this article, which is based in part on wire service reports.